strikethrough breaks editor

[cloverich]

The following markdown totally breaks my editor:

**~~~~**

-   ~~Attempt to add wikilink parsing + test~~

    -   ~~Test is ready;~~ [~~review OFM plugin~~](https://github.com/MoritzRS/obsidian-ext)

It may be related to my transient state in #265 (actually upgrading all remark libs). But in case its not, because I need to discard the contents above so I can continue using the note which it came from

[cloverich]

This is likely either:

1. Older version generating invalid markdown in some scenarios
2. Bug in my Slate / Plate text generation

Potentially this is addressed by #265 as many of these and similar examples, run as tests, don't result in invalid markdown any longer. Will leave open while I migrate from various other markdown applications as part of #247